The present invention relates to electronic compasses. More particularly, the present invention relates to compensation of electronic compasses for magnetic errors.
Electronic compasses are well known in the art. Such devices typically have used magnetic flux gate or other sensors to measure magnetic fields and to determine orientation with respect to the Earth""s magnetic field. As with needle- or card-based compasses, however, when an electronic compass is used in an environment of ferrous metals and associated perturbing fields, the fields sensed by the magnetometer sensors are incorrect, leading to erroneous readings of Earth""s magnetic field and compass azimuth.
To obtain correct readings, it is necessary to compensate for these magnetic perturbations. Compensating needle- or card-based compasses requires the use of bar magnets and/or soft iron masses to physically neutralize and cancel perturbing magnetic fields. These magnets and soft iron masses must be positioned carefully about the compass so as to cancel preexisting perturbations and reduce deviation errors to, for example, 3-5xc2x0. Even after this reduction, however, residual deviation errors must be plotted against true azimuth so that the user can correct the azimuth value.
Electronic compasses, which use microprocessors to process the data received from the magnetometer sensors, can be compensated using numerical methods. One particular example of compensation using numerical methods is the classical five-term compensation formula for a level compass. In this formula, the deviation of the azimuth as measured by the compass from the true heading is expressed in degrees as a function of the true magnetic azimuth xcex8 as follows:
Deviation=A+Bxc2x7sin(xcex8)+Cxc2x7cos(xcex8)+Dxc2x7sin(2xcex8)+Exc2x7cos(2xcex8) 
where A, B, C, D, and E are coefficients whose values are determined through some calibration procedure. This compensation technique exhibits certain limitations. For example, the above formula is only an approximate expression, valid only for small deviations and small values of A, B, C, D, and E. Thus, this technique is only used after physical compensation to reduce deviation errors, e.g., through the addition of magnets and/or soft iron masses. In addition, the formula is valid only for a level compass and is therefore poorly suited for use in environments in which the orientation of the compass may vary widely within three-dimensional space, such as heeled ships. Changes in latitude can also affect the quality of compensation. While this compensation technique is reasonable for use also with aircraft flux gate compasses, it is only approximate for pendulous flux gate compasses, as the coefficients are dependent on tilt attitude and magnetic latitude.
Certain conventional compensation techniques have been implemented, but many are limited to two-axis compasses and do not produce adequate results for a variety of arbitrary orientations of the compass in three-dimensional space. Some conventional compensation techniques have been applied to three-axis compasses. Such conventional techniques, however, fail to adequately compensate for certain types of errors because they rely on symmetric coefficient matrices.
Even after an electronic compass is compensated, certain conditions can cause the compass to lose accuracy. For example, changes in the compass or its installation, e.g., resulting from changes in the mounting of the compass with respect to the vehicle or equipment, shifts of a portion of magnetic material, or changes in magnetization of the vehicle material, can cause the internal magnetic perturbing fields to change and the magnetic compensation to become incorrect. In addition, external magnetic anomolies in the environment, such as magnetic geological formations or man-made structures, e.g., buildings or vehicles, can cause the magnetic field to become misaligned with respect to magnetic north. In this case, the compass is not accurate, even though its calibration remains valid.
Thus, both internal and external conditions can cause an electronic compass to lose accuracy even after it has been compensated. A need continues to exist to detect these conditions so that the user can recalibrate the compass or, at least, be made aware that compass readings may not be accurate.
To correct magnetic error conditions, a compensation technique is employed that mathematically corrects the measured magnetometer signals for magnetic anomalies. This technique also generates certain magnetic field values that are not used in the compensation formula, but that are used to monitor the accuracy of the electronic compass.
According to one embodiment, the present invention is directed to a method for monitoring the accuracy of an electronic compass. During calibration of the electronic compass, numerical compensation coefficients are calculated using a technique that also yields certain magnetic field values that characterize the magnetic field of the Earth during calibration. During normal operation of the electronic compass, compensated field values are subsequently used to calculate the compass magnetic azimuth. Also generated from this calculation are similar values that characterize the magnetic field of the Earth during normal operation. These values are compared with the values obtained during calibration.
In another embodiment, these characteristic values obtained during normal operation are instead compared to historical averages rather than to values obtained during calibration. This allows the accuracy to be monitored without knowledge of the values obtained during calibration.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments.